Antennas for transmission of radio signals are generally designed to match impedance as closely as possible with a transmitter so that the transmitter power output is maximized. Any difference in the impedance results in less than 100% of the potential transmitter power being transferred to and radiated by an antenna coupled to the transmitter. In contrast, antennas for reception of radio signals are generally designed to have an electric dipole series resonant frequency at or near the center of a frequency band which is to be received by a particular antenna.
At the series resonant frequency, energy is transferred from the antenna to the receiver circuit input at a maximum rate (e.g., without excessive loss). An excessive loss in this transfer occurs when there is an impedance mismatch between the antenna and the receiver circuit. Using a frequency other than the resonant frequency results in a less efficient power transfer, because the impedance mismatch between the antenna and input increases as the received frequency diverges from the resonant frequency. By "tuning" a receiving antenna to have a resonant frequency centered in the frequency band to be received, a close matching of the impedance between receiving antenna and a receiver circuit input is achieved.
At the desired resonant frequency, prior art antennas are often implemented under the constraint that the electric dipole length of a radiator must be approximately equal to half of the wavelength of a radio signal. For portable radio devices this can be a difficult problem, because the length of the antenna must be quite long for the more common transmission frequencies. For example, a traditional center fed half wavelength dipole antenna having a resonant frequency of 937.5 MegaHertz typically is approximately 16 centimeters (cm) in length. A comparable quarter-wave monopole antenna that extends above a conducting plane is 8 cm in length for nearly the same resonant frequency. In smaller portable radio devices, there is typically no space for an antenna 8 to 16 cm in length.
In order to afford an antenna which may be fitted within smaller portable devices, it thus becomes desirable to operate the antenna at a wavelength greater than eight times the dipole length. Shortening the length of dipole or monopole antennas in such manner, however, impedes its ability to exhibit the desirable series-resonant effect, especially at high operating frequencies. In theory, the shortening of an arm length of a dipole or monopole antenna reduces the inductive reactance of the antenna which in turn causes its overall reactance to become more and more capacitive. It is well known that series resonance is accomplished by designing an antenna such that a balance exists between inductive and capacitive reactance, thereby eliminating overall reactance. Since the shortening of the antenna decreases the inductive reactance, a higher capacitance must be incorporated in the antenna to reduce the capacitive reactance, thereby matching the reduced inductive reactance. In the case of short dipole or monopole antennas, the reduction of inductive reactance is severe, thus calling for a sizable increase in capacitance. This augmentation of capacitance of short dipole or monopole antennas is extremely difficult, if not impossible, in minimal volumetric spaces given present methods.
Therefore, a need exists for an antenna design which has the same electromagnetic field characteristics of a center-fed half wavelength dipole antenna, but which has a shorter length dimension. There also exists a need for a short dipole antenna that is capable of exhibiting series resonance at a frequency which corresponds to a wavelength much longer than the length of the antenna. This would allow an efficiently designed antenna to be hidden inside of the main part of a portable radio device rather than extending along the outside of the portable radio device into free space for some distance.
The present invention provides a solution to this and other problems, and offers other advantages over the prior art.